Они пришли одновременно.

Пришли is the past tense, plural form of прийти.

So in this sentence it tells you:

• the action happened in the past
• the subject is plural (they)

A rough breakdown is:

• они = they
• пришли = came / arrived
• одновременно = simultaneously / at the same time

This is a question of aspect.

• прийти → perfective
• приходить → imperfective

Пришли presents the action as a completed arrival: they arrived, and the event is seen as a whole.

Приходили would usually suggest something like:

• they used to come
• they were coming
• they came on some occasion, with focus less on the completed result

In Они пришли одновременно, the important idea is that their arrival happened at the same time, so the perfective form fits naturally.

The dictionary form is прийти.

This verb means to come or to arrive. It belongs to the group of Russian verbs of motion, and its forms are a bit irregular, so it is worth learning them carefully.

For example:

• прийти = to come
• пришёл = he came
• пришла = she came
• пришло = it came
• пришли = they came

In Russian past tense, plural forms use -ли.

Compare:

• пришёл = he came
• пришла = she came
• пришло = it came
• пришли = they came

So -ли tells you the subject is plural. In the plural, Russian past tense does not show gender.

Они is in the nominative case, because it is the subject of the sentence.

The subject is the one performing the action:

• Они = they
• пришли = came

So this is the normal subject form.

Одновременно is an adverb.

It describes how or in what way the action happened:

• they came simultaneously
• they came at the same time

Because it is an adverb, it does not change for case, number, or gender.

Not exactly.

• одновременно = simultaneously / at the same time
• вместе = together

These ideas can overlap, but they are not identical.

For example:

• Они пришли одновременно. = They arrived at the same time.
• Они пришли вместе. = They came together.

If two people arrive at the same moment but by different routes, одновременно fits. If they arrive as a pair or group, вместе fits.

Sometimes both can be true, but the focus is different.

Yes. Russian word order is fairly flexible.

You can say:

• Они пришли одновременно.
• Они одновременно пришли.
• Одновременно они пришли.

These all express roughly the same basic meaning, but the emphasis changes slightly.

• Они пришли одновременно is a neutral, natural order.
• Они одновременно пришли puts a bit more focus on simultaneously.
• Одновременно они пришли can sound more contrastive or literary depending on context.

Historically and literally, yes, прийти is connected with motion on foot.

In practice, though, Russian speakers often use it more broadly in everyday speech to mean come / arrive, especially when the exact method of travel is not important.

Still, Russian has more specific motion verbs:

• прийти = come on foot / come
• приехать = come by vehicle
• прилететь = come by flying

So if the means of transport matters, Russian may choose a different verb than English would.

Yes, often it can.

Russian frequently drops subject pronouns when the meaning is clear from context. So:

• Пришли одновременно. = They came simultaneously, if the context already makes clear who they are.

Including они makes the subject explicit. It can sound more neutral, clearer, or slightly more contrastive depending on context.

It can be translated as either, depending on context.

• They came at the same time
• They arrived at the same time

Because прийти often emphasizes reaching the destination, arrived is sometimes a little more precise. But came is also completely natural in many contexts.

So the best English choice depends on the situation, not on grammar alone.